A powder metallurgy process employing an iron-base powder to produce a product such as a sintered body is superior to other processes in terms of the cost, dimensional precision of products and productivity. Accordingly, the powder metallurgy process is widely used.
In the powder metallurgy process, a raw material powder containing an iron-base powder is mixed, followed by pressure to form a green compact, further followed by sintering at a temperature equal to or less than a melting point, whereby a sintered body is produced. Among these, a mixing step is a very important operation in view of improving the handling property of a mixed powder to improve the operation efficiency in the pressure forming step to thereby obtain a homogeneous sintered body. In the mixing step, usually, in a raw material powder in which a predetermined carbon supply component (carbon source) is added to the iron-base powder, a lubricant is added to improve the lubrication, followed by mixing.
Conventionally, as the carbon supply component, a graphite powder which is cheap and readily available is widely used.
However, when the graphite powder is used, there is a problem in that, in the mixing or pressure forming step, the graphite powder generates dust (spatter) to deteriorate the handling property of the mixed powder and a working environment. Furthermore, the graphite powder is different in a particle diameter as compared with the iron-base powder and largely different as well in the specific gravity therefrom. Accordingly, even when these are once homogeneously mixed in a mixer, during handling thereafter, separation and segregation (particle size segregation, specific gravity segregation) tend to take place.
In this connection, conventionally, as a method of inhibiting the graphite powder from segregating, a binder (bond) is used.
However, the binder usually has a tackiness and deteriorates the fluidity of the mixed powder. In the case that the fluidity of the mixed powder is poor, for example, in the pressure forming step such as when the mixed powder is exhausted from a storage hopper and sent to a forming mold or when the mixed powder is filled in a forming mold, problems that an exhaust defect owing to bridging or the like is caused at an upper portion of the exhaust of the storage hopper, and that a hose from the storage hopper to a shoe box is clogged, may occur. Furthermore, when the fluidity of the mixed powder is poor, there is another problem in that, since it becomes difficult to evenly fill the mixed powder in an entire forming mold (in particular, a thin portion), whereby it is difficult to obtain a homogeneous green compact.
In order to overcome the problems caused by the binder, patent documents 1 through 3 disclose novel binders which is capable of inhibiting the graphite powder from segregating and improving the fluidity of the mixed powder. However, when these binders are used, there are problems in that the density of the green compact cannot be sufficiently heightened and it is difficult to obtain a sintered body high in the strength and hardness.
Furthermore, in the conventional processes in which a binder is used, a step of adding a binder in the mixed powder followed by mixing is separately necessary. Accordingly, the productivity is inevitably deteriorated.
On the other hand, in patent documents 4 and 5, as the carbon supply component, carbon black is exemplified as well as graphite powder. However, in a column of examples, only the experimental results in which graphite powder is used are described and an experimental result in which carbon black is used is not at all described.
Patent document 1: JP-A 2003-105405
Patent document 2: JP-A 2004-256899
Patent document 3: JP-A 2004-360008
Patent document 4: JP-A 2004-162170
Patent document 5: JP-A 2004-115882